DEVICE FOR LIMITING OVERVOLTAGES ARRIVING AT AN OBEJCT TO BE PROTECTED VIA AN ELECTRIC HIGH-VOLTAGE TRANSMISSION LINE

Abstract: A cable (K) or other object to be protected, at which overvoltages may arrive via an electric high-voltage power transmission line (L), may be provided with an overvoltage protection means comprising two surge arresters (AL1, AL2) connected to the line, for example zinc-oxide arresters, and an inductor (X) connected into the line between the arresters.

Description: Device for limiting overvoltaaes arriving at an object to be protected via an electric hiah-voltaae transmission line TECHNICAL FIELD The present invention relates to a device for limiting overvoltages arriving at an object to be protected via an electric high-voltage power transmission line. BACKGROUND ART It has been well-known for a long time to use surge arresters as overvoltage protection means in electric highvoltage plants. For this purpose, zinc oxide varistors are preferably used. Such an arrester has a current-voltage characteristic with a "knee", with a low current at voltages below the "knee voltage" and with a very rapidly increasing current at voltages above the "knee voltage". Such an arrester is dimensioned for a certain assumed overvoltage pulse, the magnitude and form of which are determined by the network configuration and by the assumed maximally unfavourable switching operations and lightning stresses. The arrester is dimensioned so as to withstand the energy of such an overvoltage pulse without an undesired voltage - the protection level - being exceeded. At the same time, the arrester must be dimensioned so as to withstand the power development at the normal continuous operating voltage of the arrester, the so-called COV (Continuous Operating Voltage) - unlike a silicon-carbide arrester, a zinc-oxide arrester has no spark gaps but is continuously traversed by current. In the light of the above facts, it has proved that the ratio of the protection level to the continuous operating voltage cannot be lower than a lower limit value, which is substantially determined by the arrester material but also, to a certain extent, by, for example, the efficiency of the cooling of the arrester. In typical zinc-oxide arresters, thus, the value of the ratio of PL/COV cannot be lower than about 1.8 (PL = Protective Operating Level). This fact means that for a certain desired operating voltage (COV) of the arrester and the object to be protected, the latter must be dimensioned to withstand overvoltages which greatly exceed the operating voltage. Otherwise expressed, what is said above means that an object protected through zinc-oxide arresters, dimensioned to withstand a certain maximum overvoltage, cannot be driven with a continuous operating voltage which is higher than about 1/1.8 times the protection level, which in turn entails a limitation of the power handling capacity of the object to be protected. It is known that the energy handling capacity of an arrester may be increased by connecting a plurality of arrester branches in parallel. A parallel connection, however, gives rise to problems in obtaining a uniform distribution of the current between the branches. While it is true that a uniform current distribution may be obtained by selecting the arrester branches specially in relation to each other "matching" them to each other - it has proved, however, that it is possible to obtain a uniform current distribution only within a limited current or voltage range. Because of the current distribution, matching at the point which corresponds to the protection level of the arrester gives a certain, however limited, reduction of the protection level. SUMMARY OF THE INVENTION The invention aims to provide a device of the kind described in the introductory part of the description, which makes possible a reduction of the ratio of protection level to operating voltage in relation to what has been possible so far. What characterizes a device according to the invention will become clear from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail in the following with reference to the accompanying Figures 1-3, wherein Figure 1 shows the current-voltage characteristic for a typical zinc-oxide arrester. Figure 2 shows an example of a device according to the invention, and Figure 3 shows the current-voltage characteristic of the device. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows the current-voltage characteristic of a typical zinc-oxide arrester with the arrester current I on a logarithmic scale and the arrester voltage U on a linear scale. As mentioned above, COV designates the maximum continuous voltage at which the arrester may operate. PL designates the protection level of the arrester, that is, the voltage across the arrester at maximum stress. Ic is the arrester current at COV and Ip is the arrester current at the protection level. The reasoning and the designations are partially simplified. Thus, for example, the protection level PL may consist either of the so-called SIPL (Switching Impulse Protection Level), which is dependent on switching surges and which is usually dimensioning, or of the socalled LIPL (Lightning Impulse Protection Level) which is dependent on lightning surges. However, this does not affect the reasoning behind the invention and its function. For a typical zinc-oxide arrester, for example of the ABB HV Switchgear AB type EXLIM (which arrester may be used to advantage in a device according to the invention), Ic (per arrester leg) is of the order of magnitude of mA, whereas Ip may be of the order of magnitude of tens of kA. The voltages PL and COV may, depending on the number of series-connected semiconductor blocks in each leg, vary between a few kV and a few hundred kV. As mentioned, however, the ratio PL/COV is largely determined by the arrester material and in hitherto used arresters it has not been lower than a lower limit of about 1.8. Figure 2 shows a device according to the invention applied as overvoltage protection means for a cable for power transmission by means of high voltage direct current (HVDC). The cable K has a cable termination KA, at the upper end of which the cable conductor is connected to a line L which may be an overhead line or a line in a switchyard. The line L is suspended from a tower S1 with the aid of insulators I1 and I2. Between the line and ground, two zinc-oxide arresters AL1 and AL2 are connected. An inductor X is connected in the line L between the arresters. In parallel with the inductor, a third arrester AL3 is connected to limit the maximum voltage across the inductor and hence the requirements for voltage withstand capability of the inductor. In parallel with the arrester AL1, a capacitive filter is connected, which consists of a capacitor C. Between the tower S1 and a tower S2, a shield wire JL is connected for protection of the plant against direct lightning strokes. This wire is grounded via the towers. The towers, as well as the ground connections of the arresters and the filter, may be connected to a metallic grid, disposed in the ground, for reduction of the transition resistance of these plant components to ground. For the sake of clarity, Figure 2 does not show how the arresters AL1, AL2, AL3, the capacitor C or the inductor X are erected or suspended. These components may be arranged in or on insulators in a manner known per se. The plant may, for example, be designed for an operating voltage of 500 kV. In prior art plants, an arrester was arranged at the cable termination and dimensioned for a protection level of about 500 x 1.8 = 900 kV. The cable (and other plant components) must then be dimensioned to withstand overvoltages of up to this value. The most unfavourable case in a plant according to Figure 2 is often a lightning stroke in the shield wire JL, or directly on the tower S1, in which case, because of the ground impedance, the voltage at the upper end of the tower S1 may become so high as to cause a flashover from the tower to the conductor L. However, it has been found that the inductor X and the capacitor C in a device according to the invention to a great extent dampen the steep and shortlived lightning surges. The arresters in the plant according to the invention shown in Figure 2 may, as mentioned, be of the ABB HV Switchgear AB manufacture type EXLIM and be dimensioned for a protection level of 900 kV and a continuous operating voltage of 500 kV. The inductor X preferably consists of an air coil. It may suitably be a so-called line trap, that is, a blocking inductor intended for use in carrier frequency transmission on power lines. The inductor may, for example, have an inductance of 1 mH. The function of the device according to the invention is illustrated in Figure 3. At an incoming overvoltage, for example a lightning surge, the overvoltage is limited by the arrester AL1 to the protection level PL. Through the arrester, a strong current Ip flows to ground, for example of the order of magnitude of 20 kA per leg. A current also flows via the inductor X and is divided between the arrester AL2 and the cable K. A voltage drop AU is obtained across the inductor. The arrester AL2 will thus operate at the considerably lower voltage PL' = PL - AU, for example 750 kV, which entails a much lower current Ip', for example 60 A per leg. The voltage across the object to be protected, in this case the cable K, will thus be limited to the level PL', which is considerably lower than the level - PL - to which it would have been subjected in prior art devices. It has proved that the ratio PL'/COV in a device according to the invention typically may be reduced to about 1.5. This means that the requirements for voltage withstand capability of the object to be protected - the cable - may be correspondingly lower, which in turn entails a considerably lower cost of the cable. In, for example, an HVDC transmission via submarine cable, the cost of the cable constitutes a considerable part of the total cost of the plant. A reduction of the cost of the cable therefore leads to a considerable reduction of the total cost. In those cases where the object to be protected - in this case the cable - has a given voltage withstand capability, which is the case in an existing plant, the plant may be operated with a higher operating voltage than earlier by providing it with an overvoltage protection means according to the invention. The cable with a voltage withstand capability of 900 kV, described above as an example, may then be operated with an operating voltage of 900/1.5 = 600 kV, which entails a 20 per cent increase of the power transmission capacity of the plant and hence very great economic gains. The capacitor C in Figure 2 provides a low-impedance path to ground for steep overvoltage pulses and counteracts that these pulses are transferred to the cable side of the inductor via the leakage capacitance of the inductor. In the plant described above as an example, the capacitor may have a capacitance of, for example, 0.5 RF. The capacitor may be series-connected to a low-resistance resistor for limiting the capacitor current. The capacitor may possibly be omitted in certain applications. As shown in Figure 2, a surge arrester AL3 is connected in parallel with the inductor X for limiting the voltage across the inductor, thus reducing the requirement for voltage withstand capability of the inductor. If the inductor is designed with sufficient voltage withstand capability, this arrester may be omitted. The voltage after the arrester AL1 is limited by the inductor and by the arrester AL2, and therefore the former arrester may be a simpler and less expensive arrester with fewer parallel branches. Further, as is clear from Figure 3, the arrester AL1 is operating on the flatter part of the characteristic, so the reduction of the current per branch, which may be obtained by a distribution of the total current on several branches, only gives a minor reduction of the voltage of the arrester. For the arrester AL2, however, the situation is the opposite. This arrester, as is clear from Figure 3, operates on the steeper lefthand part of the characteristic. A distribution of the arrester current between several branches, that is, a reduction of the current per branch, therefore provides a considerable reduction of the arrester voltage, and hence a further reduction of the ratio PL'/COV. A uniform current distribution between the parallel branches of the arrester AL2 may simply be achieved in that "matching" of the branches only need be made at one single operating point - the point PL'- Ip'in Figure 3. In the example of a device according to the invention as described above, the object to be protected was an HVDC cable. However, the invention may be applied with, in principle, the same advantages to arbitrary types of objects and to both higher and lower voltages than those described above, and to both alternating-current and direct-current plants. In the plant described above as an example, the arresters were zinc-oxide arresters. Alternatively, however, other arresters with corresponding or similar properties may be used.

Claims: CLAIMS 1. A device for limiting overvoltages arriving at an object to be protected (K) via an electric high-voltage power transmission line (L), wherein the device comprises a first surge arrester (AL1) connected to the line, characterized in that it comprises a second surge arrester (AL2) connected to the line between the first arrester and the object to be protected, and an inductor (X) connected into the line between the arresters. 2. A device according to claim 1, characterized in that it comprises a third surge arrester (AL3), connected in parallel with the inductor (X), for limiting the voltage across the inductor. 3. A device according to claim 1 or 2, characterized in that the second surge arrester (AL2) comprises a plurality of parallel-connected branches. 4. A device according to claim 3, wherein the first surge arrester (AL1) is dimensioned for a predetermined maximum current (Ip), characterized in that the legs of the second arrester (AL2) are adapted for uniform current distribution at a current (Ip') which is considerably lower than said maximum current. 5. A device according to any of the preceding claims, characterized in that the surge arresters included in the device consist of zinc-oxide arresters. 6. A device according to any of the preceding claims, characterized in that it comprises a capacitive filter circuit (C) connected in parallel with said first surge arrester (AL1). 7. A device according to claim 6, characterized in that the filter circuit consists of a capacitor (C). AMENDED CLAIMS [received by the International Bureau on 22 May 1995 (22.05.95); original claims 1, 2, 3 amended; original claim 7 cancelled; claims 4-5 unchanged but renumbered as claims 5 and 6 (1 page)] 1. A device for limiting overvoltages arriving at an object to be protected (K) via an electric high-voltage power transmission line (L), wherein the device comprises a first surge arrester (AL1) connected to the line, a second surge arrester (AL2) connected to the line between the first arrester and the object to be protected, and an inductor (X) connected into the line between the arresters, characterized in that it comprises a capacitive filter circuit (C) connected in parallel with said first surge arrester (AL1). 2. A device according to claim 1, characterized in that the filter circuit consists of a capacitor (C). 3. A device according to claim 1 or 2, characterized in that it comprises a third surge arrester (AL3), connected in parallel with the inductor (X), for limiting the voltage across the inductor. 4. A, device according to any of the preceding claims, characterized in that the second surge arrester (AL2) comprises a plurality of parallel-connected branches. 5. A device according to claim 4, wherein the first surge arrester (AL1) is dimensioned for a predetermined maximum current (Ip), characterized in that the legs of the second arrester (AL2) are adapted for uniform current distribution at a current (Ip') which is considerably lower than said maximum current. 6. A device according to any of the preceding claims, characterized in that the surge arresters included in the device consist of zinc-oxide arresters.

Inventors: BERGDAHL BERNT, CANELHAS ANDRE, BERGDAHL, BERNT, CANELHAS, ANDRE
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1994
Main classification: H02H9
IPC codes: H01C7, H02H9